Skid control system

ABSTRACT

A system for controlling the braking of a wheeled vehicle to prevent skidding, in which the braking effect applied to the wheel is effectively responsive to one or a combination of two conditions, either a large spin-up rate of the wheel, or a small wheel speed versus car speed deviation, in this latter case it being assumed that the car speed remains essentially constant. The system generates a signal which is an approximation of the function representative of the time remaining to effectively reapply the brakes in order to anticipate the hydraulic and mechanical inertia of the system in reapplying the brake pressure to the wheels. The system includes a apparatus for sensing the wheel angular velocity, a differentiater circuit for differentiating the wheel velocity to achieve an angular acceleration signal, this acceleration signal being stored in the system for use in generating a wheel speed deviation from vehicle speed signal, and a system for sensing the rate of change of the wheel speed velocity relative to the car velocity or wheel speed deviation from vehicle speed and a second circuit for sensing the rate of spin-up of the wheel, this latter circuit including a function generator system, in the form of a nonlinear device, to vary the effect of the wheel spin-up signal rate in accordance with its magnitude. The skid control solenoid is controlled in accordance with one of these signals.

United States Patent Scharlack 1 Mar. 7, 1972 [54] SKID CONTROL SYSTEM[72] Inventor: Ronald S. Scharlack, San Antonio, Tex.

[73] Assignee: Kelsey-Hayes Company, Romulus, Mich.

[22] Filed: Nov. 12, 1969 [21] Appl. No.: 875,993

[52] US. Cl. ..303/2l P, 303/20 [51] Int. Cl. ..B60t 8/08, B60t 8/12[58] Field oiSearch ..l88/l8l;303/21,20;317/5; 324/ 160-162; 340/262263[56] References Cited UNITED STATES PATENTS 3,245,213 4/1966 Thompson etal. ..303/21 BB 3,467,443 9/1969 Okamoto et a1. ...303/2l BB 3,494,6712/1970 Slavin et a1. ..303/2l P 3,498,682 3/1970 Mueller et al. 303/21BB 3,499,689 3/ 1970 Carp et a1 .303/21 P 3,511,542 5/1970 Fielek, Jr..303/21 P 3,520,575 7/1970 Steigerwald... .303/21 BB 3,525,553 8/1970Carp et al ..303/2l P Primary Examiner-Milton Buchler AssistantExaminer-Stephen G. Kunin Attorney-Harness, Dickey & Pierce [5 7]ABSTRACT A system for controlling the braking of a wheeled vehicle toprevent skidding, in which the braking effect applied to the wheel iseffectively responsive to one or a combination of two conditions, eithera large spin-up rate of the wheel, or a small wheel speed versus carspeed deviation, in this latter case it being assumed that the car speedremains essentially constant. The system generates a signal which is anapproximation of the function representative of the time remaining toeffectively reapply the brakes in order to anticipate the hydraulic andmechanical inertia of the system in reapplying the brake pressure to thewheels. The system includes a apparatus for sensing the wheel angularvelocity, a differentiater circuit for differentiating the wheelvelocity to achieve an angular acceleration signal, this accelerationsignal being stored in the system for use in generating a wheel speeddeviation from vehicle speed signal, and a system for sensing the rateof change of the wheel speed velocity relative to the car velocity orwheel speed deviation from vehicle speed and a second circuit forsensing the rate of spin-up of the wheel, this latter circuit includinga function generator system, in the form of a nonlinear device, to varythe effect of the wheel spin-up signal rate in accordance with itsmagnitude. The skid control solenoid is controlled in accordance withone of these signals.

10 Claims, 3 Drawing Figures mesa spam j sin/50k SKID CONTROL SYSTEMBACKGROUND AND SUMMARY OF THE DISCLOSURE This invention relatesgenerally to vehicle braking systems and, more particularly, to a brakeand skid control system for preventing wheel skidding and minimizingstopping distances while simultaneously maintaining directionalstability through the control of the application of braking pressure inresponse to the spin-up rate of the wheel or, in the alternative, thesensing of a small wheel speed deviation from vehicle speed.

One of the major difficulties which arises in braking a moving vehicle,such as an automobile, an aircraft or other wheeled vehicle, occurs whenthe braked wheel, or wheels, lock up. This lock up condition tends tocreate an unstable situation in the controlled motion of the vehicle. Atthe same time, a locked wheel condition generally increases stoppingdistance.

Several skid control systems have been evolved which provide maximumefficiency under various road conditions while utilizing a relativelysimple computational system. One such system, disclosed in copendingapplication of David B. Eisenhaure and Ronald S. Scharlack, Ser. No.626,626, filed Mar. 28, 1967, now U.S. Pat. No. 3,508,795, issued Apr.28, 1970 takes into account the changing road conditions which result ina change in the coefficient of friction. In this system, the linear andangular accelerations of the braking wheel, or wheels, are sensed byappropriate accelerometer devices. The output signals from suchaccelerometers are fed to a simple analog computer system which producesoutput signals proportional to the rate of change of the braking forceas a function of time and to the rate of change of the wheel slip as afunction of time.

Another system has been evolved which eliminates the requirement ofaccelerometer devices for sensing the linear and angular acceleration ofthe braking wheel and substitutes, in lieu thereof, a single angularvelocity sensing device for providing a condition signal which isindicative of the angular wheel velocity. This condition signal providesall of the information necessary to effectively operate the skid controlsystem of the invention disclosed in a second copending application ofRonald S. Scharlack, Ser. No. 769,035, filed Oct. 21, 1968, now U.S.Pat. No. 3,532,392, issued Oct. 6, 1970 and accomplishes essentiallymaximum efficiency and optimum operation of the braking system. In thissystem, it is recognized that the first derivative of the brake force isproportional to the rate of change of acceleration, after eliminatingthe effectively constant wheel mass. Thus, to maximize the brakingforce, (that is, optimum slip on the brake force versus slip curve), therate of change of acceleration must be at a zero point when the wheel isaccelerated, this point being at the point of maximum brake force. Inthis system, the deceleration signal generated within the control systemis monitored and the brakes are triggered to the on" condition when apreselected deceleration is reached which is indicative of an incipientskid condition. Also, a further system utilizing the just describedfeatures and principles, is disclosed in third copending application ofHugh E. Riordan, Ser. No. 7,711,531, filed Oct. 29, 1968, now U.S. Pat.No. 3,532,393, issued Oct. 6, 1970. In this latter system, the circuitryhas been reduced to further simplify and reduce the cost of previoussystems.

The foregoing applications are cited herein to incorporate discussionsof the operation of a braked wheel and the effects of varying the forceapplied to a braked wheel on the skidding tendencies of the wheel. Thesystem of the present invention utilizes a different mode of operationin controlling the application of the brakes or braking force to a wheelby sensing the wheel angular velocity and deriving two signalsrepresentative of conditions of the spin-up of an unbraked wheel. Thefirst of these conditions is a relatively low-wheel deviation relativeto a previously defined reference signal. The second condition sensed isthe spin-up rate of the wheel upon release of the braking force fromthat wheel. The effect of this latter signal on the control device isvaried in accordance with some preselected function which is responsiveto the magnitude of spin-up rate. In the disclosed embodiment, thisfunction circuit takes the form of a resistor in parallel with aresistor-zener diode combination. This circuit approximates the timeremaining, a variant of the spin-up rate, to reapply brake pressurebefore the actual force is applied to the wheels. These two signals areutilized to control the reapplication of the brakes to the wheel as itis spinning-up, one of the conditions being a preset high rate of wheelspin-up and the other condition being a preset low wheel speed deviationas compared to car speed deviation, this latter condition beingpreviously defined in the circuitry.

Accordingly, one object of the present invention is to provide animproved controlled braking system for a vehicle;

It is another object of the present invention to provide an improvedskid control system for the brake or brakes of a wheeled vehicle;

It is still another object of the present invention to provide a brakecontrol system of the type described which controls the reapplication ofthe brakes in accordance with two sensed conditions of the wheelspin-up;

It is still a further object of the present invention to provide animproved skid control system for the brakes of a vehicle which sensesthe rate of spin-up of the wheel and the wheel deviation from a fixedpoint and correlates these two signals to control the reapplication ofbrake force;

It is another object of the present invention to provide an improvedskid control system of the type described which operates to reapplybraking force in response to a sensed high rate of spin-up of the wheelor a sensed low wheel deviation relative to a fixed signal;

It is still a further object of the present invention to provide animproved skid control system for the brakes of a vehicle which is simpleand inexpensive to manufacture and install and is reliable in use;

It is still another object of the present invention to provide animproved anticipation circuit for varying the effect of the spin-up ratesignal on the output control device in accordance with the anticipatedinertia of the system which delays the actual'application of the brakeforce to the wheels after reapplication of brake pressure.

Further objects, features and advantages of this invention will becomeapparent from a consideration of the following description, the appendedclaims and the accompanying drawings in which:

FIG. 1 is a schematic diagram illustrating a preferred circuit forcontrolling the reapplication of braking force to the wheels of thevehicle;

FIG. 2 is a schematic diagram illustrating certain details of the systemof FIG. 1; and

FIG. 3 is a schematic diagram illustrating a modification of a portionof the system of FIGS. 1 and 2.

The skid control system of the present invention is particularly adaptedto be utilized and will be described specifically for use in anautomotive vehicle. However, as was stated above, it is to be understoodthat the features of the invention may be utilized with other types ofvehicles including aircraft and other wheeled vehicles which are adaptedto provide a braking force through a wheel type of element. In the caseof an automotive use, the system of the present invention may beutilized in connection with either the front wheels, the rear wheels orboth the front and rear wheels. The same use occurs in other vehicleswhich utilize longitudinally displaced braking wheels. However, forsimplicity, the system will be described for.use in conjunction onlywith the rear wheels of an automotive vehicle.

The system of the present invention generally includes a capacitor whichhas a zero charge when the wheels are traveling at the same rate as thevehicle, no brake force being applied. When the solenoid is applied, thecapacitor is charged at a rate which is a function of the deviation ofthe wheel speed from the vehicle speed or the rate of wheeldeceleration. Thus,

the charge on the capacitor is proportional to the deviation of thewheel speed from the vehicle speed at any time in the cycle.Subsequently, as the wheel is spinning up the charge on the capacitor isdrained off at a rate corresponding to the rate of change of wheelvelocity. Thus, a signal is developed at the end of the first cyclewhich is proportional to the velocity deviation of the wheel relative tothe car velocity, it being assumed that the car velocity remains arelative constant for each cycle. The charge on the capacitor is resetduring each cycle to approximate the decreasing vehicle velocity. Thus,this voltage on the capacitor is increased during the period that thebrake force is applied and, when the force switches from applied to notapplied and the wheel is still decelerating. When the wheel reaches theminimum, the charging process is terminated and the charge applied tothe capacitor is related to the maximum deviation point of the wheels asrelated to the vehicle velocity at the start of the cycle.

The aforementioned current drain from the capacitor occurs when thewheel begins the acceleration portion of the cycle or the spin-up, thecurrent being drawn from the capacitor at a rate which is proportionalto the wheel acceleration so that the voltage on the capacitorcorresponds to the deviation of the wheel from the velocity at the startof the cycle being considered. This voltage on the capacitor at anyinstant in the cycle corresponds to the wheel velocity deviation fromthe vehicle velocity at the start of that particular cycle and is theintegral of the wheel deceleration and acceleration curve, the capacitorbeing an integrating device. It is again noted that the charge on thecapacitor is reset to zero at the point of reapplication of the brakesto ready the circuit for the start of the next cycle. The voltage on thecapacitor is sensed or read through a high-impedance device to insurelow leakage, this voltage being fed to a comparator circuit which maytake the form of a single transistor or a differential amplifiercircuit.

The acceleration or spin-up signal is also fed to a second leg of thecontrol circuit, the second leg including a nonlinear device generatinga two-stage spin-up rate sensing circuit. In this connection, it is tobe noted that the brake system of a vehicle has certain inertia whichintroduces a delay between the application of the brake pressure and theactual appearance of thebrake force at the wheels. This inertia or delaybecomes more critical as the rate of spin-up of the wheel increases andit becomes necessary to further anticipate the point of reapplication ofthe brake force by reapplying the brake pressure earlier in the wheelrun down and spin-up cycle. In this way, variations in surfaceconditions, highpt as compared to Iowa, may be accommodated.Accordingly, it becomes necessary to provide a circuit which givesgreater than a straight line effect to the spin-up rate signal at higherspin-up rates. In the circuit of the present invention, a zener diode isused to sense the spin-up rate, and when the rate is large enough, tointroduce a greater slope to the spin-up rate versus time signalfunction. It is to be understood that only two ratesare disclosed forillustrative purposes, i.e., a resistor which is linear in efiect anda'zener diode and resistor corribination which is nonlinear in effect.The circuit could be provided with several more parallel legs with zenerdiodes having different avalanche characteristics. On the other hand,other function generator circuits, to approximate any desired function,may be inserted between the spin-up rate signal and the output device tovary the effect of the spin-up rate signal on the output.

Thus, a voltage is generated which is proportional to the time delayneeded for the pressure to reach the level required to counteract thevariations in acceleration rate of the wheels from one surface toanother. The two signals are brought together at the above-describedsingle comparator device or the difi'erential circuit to provide anoutput signal which is controlled by or responsive to one of twoconditions, either the charge on the capacitor indicating a low wheeldeviation condition or the signal from the other leg indicating a highspin-up rate condition. Both conditions may be variably adjusted toprovide control at any point in the operation of the braking system.When the conditions are such that one or the other of the two signalscreates an output signal from the comparator circuit, the output signalis used to turn off the solenoid and arrest the wheel spin-up. Thus, thesystem starts into the second portion of the cycle. At the time thesolenoid is turned off, the capacitor is short circuited to dischargethe capacitor and reset the circuit.

Referring now to FIG. 1, there is illustrated a schematic block diagramof a control system 10 which may be utilized to control the operation ofthe brake force applying solenoid in accordance with the featuresdiscussed above. The system includes a transducer 12 for sensing thewheel angular velocity omega (0:), the output signal of which is fed toa negative differentiator circuit 14 by means of a conductor 16. Theoutput of the differentiator circuit 14 produces a signal on conductor18 which is the negative of the rate of change of wheel velocity ofwheel acceleration. This signal on conductor 18 is fed to a negativeamplifier and integrator circuit 33 which, on the rundown portion of thewheel velocity signal, produces an output signal which is a function ofthe wheel velocity signal. When the spin-up portion of the wheel cyclecommences, which is achieved onthe lowest point of the wheel speed waveform, the integration of the lheel velocity signal ceases. Theintegrated signal is stored within the circuit 33.

The circuit 33 also includes a circuit which changes the stored signalin accordance with a particular function of the spin-up portion of thecycle. Ultimately, the stored signal at any instant of time is afunction of the deviation of the wheel velocity from the vehiclevelocity at the start of that particular cycle. As will be seen from afurther description of the system, the brakes are reapplied as afunction of a low-wheel speed deviation from vehicle speed signal,either taken alone or in combination with a signal to be described whichincreases the effect of the spin-up rate signal when a preselected ratehas been reached.

Referring now to the lower portion of FIG. 1, it is seen that the signalon conductor 80 is also fed to a nonlinear circuit 82 which, in theillustrated embodiment includes a zener diode 84 connected in serieswith a resistor 86. the series combination 84,86 being connected inparallel with a resistor 88. As is noted from the dotted lines,additional circuits may be added to vary the effect of the spin-up rateon the output circuit in accordance with any desired function. Theoutput of the circuit 82 is fed to an operational amplifier 94, thepositive output of which is grounded and the negative output includes afeedback signal from the output of the operational amplifier to anegative input thereof by means of a resistor 100.

The outputs from the integrator circuit 33 and the operational amplifier94 are fed to a comparator circuit 57 which I utilizes a combination ofthe low-wheel speed deviation from vehicle speed signal from theintegrator circuit 33 and a high spin-up rate signal, as modified by thenonlinear circuit 82, to produce an output signal on conductor whichoutput signal is utilized to reapply braking force to the wheel. Thereapplication of the braking force is also utilized to reset theintegrator circuit 33 by means of an input to the conductor 136.

Referring now to FIG. 2 of the drawings, there is illustrated specificdetails of the control system 10 described in conjunction with thedescription of FIG. 1 which is adapted to control theoperation of thesolenoid and apply or reapply the brakes, and particularly to turn thesolenoid ofi at preselected conditions of the wheel velocity andacceleration. The system includes a transducer 12 for sensing the wheelangular velocity, the output signal to being fed to a differentiatorcircuit 14 by means of a conductor 16. The output of the differentiatorcircuit 14 produces a signal on conductor 18 which is the negative ofthe rate of change of wheel velocity, or wheel acceleration. The signalon conductor 18 is amplified and inverted through an amplifier circuit20 to produce the rate of change of the velocity signal or a positiveacceleration signal, this signal being fed to the main circuit 10 bymeans of a conductor 24.

As is seen from the above mentioned copending applications, upon theapplication of the braking force, the wheels will start to decelerateuntil a predetermined deceleration is reached. At this time, the brakeswill be released by means of actuating the solenoid in the skid controlsystem, the solenoid being interposed between the master cylinder andthe wheel cylinders to remove the effect of the braking force when thesolenoid is applied and reapply the braking force when the solenoid isdeenergized.

This wheel deceleration signal is fed to a transistor 30, andparticularly the base circuit thereof, by means of a resistor 32. Thesignal is initially applied to the base of transistor 30 at a time whenthe wheel velocity starts to decrease and maximum signal occurs at themaximum negative slope of the wheel velocity signal. This signal is fedto a capacitor 34 through the conduction of the transistor 30. In thepreferred embodiment, the base electrode of transistor 30 is biased suchthat the collector current is zero when the acceleration signal is zero.Thus, the capacitor 34 is provided with an initial charge of zero. Oneside of the capacitor is connected to ground by means of conductor 36.

The voltage on the capacitor 34 is sensed by means of a Darlingtoncircuit 40 which includes an emitter-follower connected with transistor42 having a collector electrode connected to a positive 8.2 volts atinput terminal 44, and an emitter electrode connected to the basecircuit of a second transistor 46. The output of transistor 46,particularly the emitter electrode thereof, is fed from a node 56 to thecollector electrode of a transistor 58 by means of a conductor 60 and aresistor 62. The transistor 58 forms the device which senses andcompares the two signals described above, one being the wheel deviationsignal and the other being the rate of change of wheel velocity signal.

The wheel velocity signal is fed to a conductor 24 and is sensed bymeans of an input transistor 70 which has a collector electrodeconnected to a positive 8.2 voltage potential at input terminal 68through the emitter-collector circuit of transistor 30, and the emitterelectrode is connected to ground. This connection will be explainedhereinafter in connection with the description of the voltage developedon capacitor 34. The voltage on conductor 80 is fed to the nonlinearcombination circuit 82 which includes a zener diode 84 connected in aseries with a resistor 86, the series combination 84,86 being connectedin parallel with a resistor 88. This combination circuit (resistors 88and 86, and zener diode 84) are used to vary the effect of the spin-uprate signal on the transistor 58, as conducted through transistor 94 inaccordance with the discussion above. The output of the combination 82is fed to node 90, the voltage at node 90 following a preselected slopeas the current builds up until such time that the zener breakdownvoltage is achieved. At this time, the slope will increase due to thelower impedance of the combination circuit 84,86. The voltage at thenode 90 is fed to a transistor 94, the transistor 94 including acollector electrode connected to a positive source of potential througha resistor 96 and the emitter electrode being grounded at 98. Also, afeedback circuit is formed by means of resistor 100. The'output oftransistor 94 is fed to the base electrode of transistor 58 by means ofa conductor 104 and a resistor 106.

The combined output of transistor 58 is fed to a transistor 118 by meansof a conductor 120 and a resistor 122. The transistor 58, whenconductive, drops the voltage on the conductor 120 down to substantiallyground potential or approximately 9% volt. The conductivity oftransistor 58 is governed by the signal impressed on conductor 104 andresistor 106 in response to the sensing of the rate of acceleration orrate of spin-up of the wheel. n the other hand, the voltage of conductor120, in the absence of the conducting of transistor 58, follows thevoltage output from the emitter electrode of transistor 46 in responseto the wheel deviation signal. Thus, when the wheel deviation signaldrops to a preselected low value, the signal on conductor 120 also dropsto a low value. With a low-voltage signal on conductor 120, thetransistor 118 will be cut off to provide a flow of current from apositive 8.2 volt potential at input terminal 124, through resistor 126,to an output terminal 130. The output terminal 130 is connected to thesolenoid valve of the skid control system. In order to turn the solenoidoff, the voltage on conductor 120 must be increased to turn transistor118 on.

The signal from the solenoid is also fed to a transistor 134 having anemitter electrode connected to the capacitor 34 and a collectorelectrode connected to ground. The emitter-collector circuit oftransistor 134 is connected in series with the capacitor 34 to shunt thecapacitor when the solenoid is turned on.

In operation, the output signal on conductor 24 is fed to the transistor30, this signal varying in accordance with the rate of change of wheelvelocity. This signal is amplified through the transistor 30, and fed tothe capacitor 34 to charge the capacitor during the period when thewheel is decelerating with a voltage which is proportional to thedeceleration signal on conductor 24. When the spin-up of the wheelcommences, that is after the achieving of the lower point of the wheelspeed wave form, the voltage on the conductor 24 reverses in polarity toturn the transistor 30 off. Thus, the integrated signal from theconductor 24 is stored in capacitor 34.

The wheel acceleration signal is also fed to the transistor 70 connectedin parallel with the capacitor 34. Thus, as the wheel commences thespin-up portion of the cycle, the conductivity of transistor 70 startsfrom zero and increases as the rate of spin-up increases to increase therate of discharge of the capacitor 34. Accordingly, the current drawnfrom the capacitor 34 is proportional to the rate of spin-up of thewheel. The charge on the capacitor at any instant is a function of thedeviation of the wheel velocity from the vehicle velocity at the startof that particular cycle. The signal on capacitor 34 is sensed by theDarlington circuit 40, and is then fed to the output conductor 120 bymeans of conductor 60 and resistor 62. When the signal drops to apreselected low voltage. the transistor 118 is turned off to provide ahigh voltage on output terminal 130. The high voltage causes thesolenoid to turn on in response to the nonconductivity of transistor118. This solenoid signal is then fed to the transistor 134 to resetcapacito 34.

The aforementioned circuit describes the operation of the system when apreselected low-wheel deviation signal is sensed. On the other hand, ifa high wheel acceleration is sensed, this signal is fed through acircuit to cause conduction of the transistor 58 to lower the voltage atthe output conductor 120 irrespective of the voltage at conductor 60.The parallel combination circuit 82 has been provided to increase theeffect of the spin-up rate signal when a preselected rate has beenreached, for example, a voltage corresponding to the zener breakdownvoltage of the zener diode 84. In this way, the impedance of the circuitto the transistor 94 is decreased to increase the effect of the signalon conductor 80.

Referring now to FIG. 3, there is an alternative embodiment of thecircuit of the lower right hand portion of the circuit of FIG. 1.Particularly, the circuit of FIG. 3 illustrates the modification whichmay be substituted for the transistor 58, the conductor 120, theresistor 122, transistor 118 and the remaining circuit connected totransistor 118. The system includes a differential amplifier circuit140, having a pair of transistors 142 and 144 connected in theconventional differential amplifier configuration. The emitterelectrodes of transistors 142 and 144, are connected to a source ofpositive potential at input terminal 146 through a resistor 148. Thecollector electrode of transistor 142 is grounded at 150 and thecollector electrode of transistor 144 is connected to ground at 154through a resistor 156. The input signals from the wheel speed deviationfrom vehicle speed portion of the circuit are fed to the base circuit oftransistor 144 by means of the resistor 62. Similarly, the nonlinearfunction of the wheel acceleration is fed to the base electrode oftransistor 142 through the resistor 106.

The output of the differential amplifier circuit 140 is fed to an outputtransistor 160 by means of a conductor 162. The transistor 160 isconnected in a similar arrangement to that described in conjunction withthe transistor 118 whereby the voltage at an output terminal 166 iscontrolled by the conduction of transistor 160. Accordingly, if thenonlinear function of the acceleration rises to a preselected level, theoutput of the differential circuit 140 will lower the voltage being fedto the conductor 162 to turn the transistor 160 off. Similarly, if thewheel deviation signal and input resistor 62 drops to a low level, theconductivity of transistor 144 will be increased to turn on transistor160. In either event, the output voltage signal at conductor 166 turnsthe solenoid on to enable the brake pressure or braking force to takeeffect.

While it will be apparent that the preferred embodiments of theinvention disclosed are well calculated to fulfill the objects abovestated, it will be appreciated that the invention is susceptible tomodification, variation and change without departing from the properscope or fair meaning of the subjoined claims.

What is claimed is:

l. A skid control system for use in controlling the braking forceapplied to at least one wheel of a vehicle, the system including asolenoid controlled valve for controlling the application of the brakingforce to the wheel and a transducer for sensing wheel velocity andgenerating a signal which is a function of the angular velocity of thewheel, skid detector means for detecting the occurrence of an incipientskid condition and supplying a brake pressure release signal to saidsolenoid controlled valve to thereby alleviate said skid condition, theimprovement comprising means for reapplying the braking force to thewheel upon alleviation of said incipient skid condition including meansfor generating a reference deviation signal which represents thestarting point of the wheel speed at a preselected portion of thebraking cycle, means for generating a wheel deviation signal whichdeviates from said reference deviation signal and varies as a functionof the difference between the speed of the vehicle and the speed of thewheel, output control means responsive to a preselected minimumlow-wheel deviation signal for controlling the solenoid valve, and meansresponsive to the rate of spin-up of the wheel for varying the effect ofsaid wheel deviation signal on said output control means, saidlast-named means including means for varying the variation in effect ofthe deviation signal in response to variations in spin-up rate of thewheel.

2. The improvement of claim 1 wherein said reference deviationsignal-generating means includes a signal level storage circuit, saidstorage circuit being connected to store a preselected function of saidangular velocity.

3. The improvement of claim 2 further including means for generating arate of change of angular velocity signal in response to said angularvelocity signal, said stored signal being said reference deviationsignal.

4. The improvement of claim 3 further including means for varying saiddeviation signal in response to variations in said angular wheelvelocity.

5. The improvement of claim 4 wherein said minimum lowwheel deviationsignal switches said solenoid valve from a first state to a secondstate.

6. The improvement of claim 5 further including resetting circuit meansconnected to said storage means and wherein the switching of saidsolenoid valve from one state to the other 7 state resets said storagecircuit.

7. In a skid control system for use in controlling the braking forceapplied to at least one wheel of a vehicle, the system including asolenoid controlled valve for controlling the application of brakingforce to the wheel and a transducer for sensing wheel velocity andgenerating a signal which is a function of the angular velocity of thewheel skid detector means for detecting the occurrence of an incipientskid condition and supplying a brake pressure release signal to saidsolenoid controlled valve to thereby alleviate said skid condition, theimrovement comprisin means for rea plying the braking force 0 the wheelupon al eviation of sar incipient skid condition including means forsensing the rate of change of wheel velocity, means for sensing thewheel velocity deviation from the vehicle velocity, output circuit meansfor controlling the valve in response to said wheel velocity deviationand said rate of change of wheel velocity, nonlinear means connected tosaid rate-sensing means for varying the effect of the ratesensing signalon the output circuit in response to the rate of change of spin-up as anonlinear function during the spin-up portion of the brake cycle, saidnonlinear means varying the variation in effect of the rate sensingsignal in response to variations in the spin-up rate of the wheel, andmeans responsive to said nonlinear circuit for controlling said solenoidin response to one of said linear signals achieving a preselected value.

8. The improvement of claim 7 wherein said nonlinear means senses thespin-up rate to generate and anticipation signal to anticipate the delaybetween reapplication of the brake pressure and brake force.

9. The improvement of claim 8 wherein said nonlinear means generates afirst linear signal in response to a first rate of change of wheelvelocity signals and generating a second signal, different from saidfirst signal in response to a second range of rate of change signals.

10. The improvement of claim 9 wherein said second signal is linear.

1. A skid control system for use in controlling the braking forceapplied to at least one wheel of a vehicle, the system including asolenoid controlled valve for controlling the application of the brakingforce to the wheel and a transducer for sensing wheel velocity andgenerating a signal which is a function of the angular velocity of thewheel, skid detector means for detecting the occurrence of an incipientskid condition and supplying a brake pressure release signal to saidsolenoid controlled valve to thereby alleviate said skid condition, theimprovement comprising means for reapplying the braking force to thewheel upon alleviation of said incipient skid condition including meansfor generating a reference deviation signal which represents thestarting point of the wheel speed at a preselected portion of thebraking cycle, means for generating a wheel deviation signal whichdeviates from said reference deviation signal and varies as a functionof the difference between the speed of the vehicle and the speed of thewheel, output control means responsive to a preselected minimumlow-wheel deviation signal for controlling the solenoid valve, and meansresponsive to the rate of spin-up of the wheel for varying the effect ofsaid wheel deviation signal on said output control means, saidlast-named means including means for varying the variation in effect ofthe deviation signal in response to variations in spinup rate of thewheel.
 2. The improvement of claim 1 wherein said reference deviationsignal-generating means includes a signal level storage circuit, saidstorage circuit being connected to store a preselected function of saidangular velocity.
 3. The improvement of claim 2 further including meansfor generating a rate of change of angular velocity signal in responseto said angular velocity signal, said stored signal being said referencedeviation signal.
 4. The improvement of claim 3 further including meansfor varying said deviation signal in response to variations in saidangular wheel velocity.
 5. The improvement of claim 4 wherein saidminimum low-wheel deviation signal switches said solenoid valve from afirst state to a second state.
 6. The improvement of claim 5 furtherincluding resetting circuit means connected to said storage means andwherein the switching of said solenoid valve from one state to the otherstate resets said storage circuit.
 7. In a skid control system for usein controlling the braking force applied to at least one wheel of avehicle, the system including a solenoid controlled valve forcontrolling the application of braking force to the wheel and atransducer for sensing wheel velocity and generating a signal which is afunction of the angular velocity of the wheel skid detector means fordetecting the occurrence of an incipient skiD condition and supplying abrake pressure release signal to said solenoid controlled valve tothereby alleviate said skid condition, the improvement comprising meansfor reapplying the braking force to the wheel upon alleviation of saidincipient skid condition including means for sensing the rate of changeof wheel velocity, means for sensing the wheel velocity deviation fromthe vehicle velocity, output circuit means for controlling the valve inresponse to said wheel velocity deviation and said rate of change ofwheel velocity, nonlinear means connected to said rate-sensing means forvarying the effect of the rate-sensing signal on the output circuit inresponse to the rate of change of spin-up as a nonlinear function duringthe spin-up portion of the brake cycle, said nonlinear means varying thevariation in effect of the rate sensing signal in response to variationsin the spin-up rate of the wheel, and means responsive to said nonlinearcircuit for controlling said solenoid in response to one of said linearsignals achieving a preselected value.
 8. The improvement of claim 7wherein said nonlinear means senses the spin-up rate to generate andanticipation signal to anticipate the delay between reapplication of thebrake pressure and brake force.
 9. The improvement of claim 8 whereinsaid nonlinear means generates a first linear signal in response to afirst rate of change of wheel velocity signals and generating a secondsignal, different from said first signal in response to a second rangeof rate of change signals.
 10. The improvement of claim 9 wherein saidsecond signal is linear.